In integrated circuit packages, the electronic components are mounted on a substrate, circuit board or carrier. The electrical connection between the components and the substrate can be achieved through wire bonds, or through connecting bumps, such as solder bumps. The chip, substrate and interconnection are typically encapsulated to produce the final package.
In Flip Chip packages, also known as Direct Chip Attach packages, the electronic components are directly connected to the substrate, circuit board or carrier by means of conductive bumps on the chip bond pads. To achieve this, the chip is normally turned face down (i.e., flipped) for connection to the substrate.
As well as providing the direct electrical connection between the chip and substrate, the bump provides a thermally conductive path for carrying heat from the chip to the substrate, as well as providing part of the mechanical mounting of the die to the substrate. The bump also acts to space the chip from the substrate, thereby preventing electrical contact between the chip and the substrate.
In Flip Chip assemblies, a non-conductive underfill adhesive is provided under the chip filling the void between the chip and the substrate. This underfill layer acts to protect the bumps from moisture or other environmental hazards, and provides additional mechanical strength to the assembly. Importantly, the underfill mechanically locks together the chip and substrate so that differences in the thermal expansion of the chip and substrate do not break or damage the electrical connection of the bumps. Accordingly, the underfill layer is important in maintaining the integrity of the package, and especially to maintain the integrity of the package despite the ambient conditions.
One problem with known Flip Chip packages is their susceptibility to moisture attack, and in particular “popcorn-delamination” whereby moisture can cause the delamination of the chip, underfill layer and substrate. Such delamination can result in the failure of the electrical connection between the chip and substrate.
It is currently considered that the moisture performance of packages is dependent upon the materials used for the flux, underfill and mold compounds, and to the adhesion between the die and substrate. However, there is a limit in the improvements that can be achieved merely by selection of materials and improvement of the adhesion.